An insulated circuit board used in a semiconductor power module has a structure in which a circuit electrode is joined to one main surface and the other main surface of an insulated substrate made of ceramic. With the recent increase in current density and higher temperature operation of power modules, it has been considered to increase the thickness of the circuit electrode for the purpose of reducing electrical resistance and thermal resistance.
If the thickness of a circuit electrode is increased, when a thermal shock cycle is applied to a power module including this circuit electrode, breakage of an insulated substrate and peeling off of the circuit electrode may occur due to thermal stress caused by a difference in thermal expansion coefficient between the insulated substrate and the circuit electrode.
To address such issues that may arise due to the increased thickness of a circuit electrode, Japanese Patent Publication No. 5-25397 (PTD 1), for example, proposes a technique in which an outer edge portion of a circuit electrode is formed to have a stepped shape, so that the circuit electrode has a smaller thickness at this stepped-shaped portion than at a portion other than the stepped-shaped portion. Japanese Patent Laying-Open No. 8-274423 (PTD 2), for example, proposes a technique in which a plurality of discontinuous trenches are provided in the vicinity of an outer edge portion of a circuit electrode.